The present invention relates generally to paper handling and sheet handling equipment. More particularly, the invention relates to a sheet feeding sucker assembly and method of operation which provides a high degree of accuracy and reliability through the use of electrical vacuum sensing control. The sheet feeder has bins with space saving inclined walls which permit a staggered feeding sequence and an analog optical misfeed detection circuit detects when sheets are missing or when multiple sheets stick together.
Sheet feeding mechanisms are used in a wide variety of printing machines, copying machines, collating machines, binding machines and the like. Typically, the sheet feeding mechanism uses vacuum or suction to transport a sheet from one position to another. For example, often a sheet feeding mechanism is provided with a stack of sheets to be fed and a sucker apparatus is used to lift an individual sheet from the stack and move it to a different station for further processing. One or more suction cup devices are used for this purpose. The suction cups are moved into contact with the surface of the sheet to be fed and sufficient suction or vacuum is applied to the suction cup, causing the sheet to stick to it.
When thin or porous sheets are being fed, sometimes the suction will bleed through to the next sheet on the stack, causing two sheets (or more) to be fed instead of only one. This is becoming increasingly problematic as there is increased use of recycled paper, which has a wider range of textures and porosities and which tends to be more difficult to feed accurately. Further, the printing industry is using a wider variety of different paper surfaces and printing inks today. These also make it more difficult to feed accurately, a single sheet at a time.
Prior sheet feeding mechanisms have not adequately addressed the problem of handling different sheet thickness and porosity. Conventional sheet feeding equipment is not sensitive enough, for example, to handle a variety of different sheet thicknesses and porosities without undergoing complicated mechanical recalibration. Conventional equipment, in general, requires a considerable amount of operator adjustment or fine-tuning to work with a given paper stock. A different adjustment and further fine-tuning is often required when the paper stock changes. Some papers, such as some recycled papers, simply have too wide a range of surface texture and porosity to work well with conventional feeders.
The present invention addresses the problem of how to handle different sheet thicknesses and porosities. The invention is readily calibrated or adapted to a wide range of different thickness and porosities; and calibration can be done without a lengthy mechanical retrofitting procedure. The present invention provides a high degree of accuracy and sensitivity to the actual vacuum established when the sheet is being picked up. In contrast with the passive triggering mechanisms of prior art devices, the invention uses highly accurate electrical pressure sensing equipment which allows the invention to lift a single sheet of comparatively porous material before the vacuum level rises to the point where multiple sheets will be adhered.
Accordingly, the invention provides a sheet sucker for a sheet feeding apparatus having a hopper or bin for containing a stack of sheets to be fed and a movable carriage supporting at least one sucker member. A source of suction applies a vacuum to the sucker member and a movement mechanism is provided for selectively moving the carriage in an engaging direction, by which the sucker member is brought into contact with a sheet on the stack of sheets to be fed. The movement mechanism also selectively moves in a retracting direction, by which the sucker member moves away from the stack of sheets to be fed.
Further in accordance with the invention, a control means is coupled to the movement mechanism for selecting the direction of movement. The control means includes electrical or electronic vacuum pressure sensing apparatus which communicates with the sucker member. The control means also includes a readily calibrated electrical or electronic set point comparing device which is coupled to the sensing means for initiating movement in the retracting direction, after the vacuum pressure at the sucker member attains a predetermined level adequate to hold a sheet in contact with the sucker member during movement in the retracting direction.
Aside from the sheet feeding problem, conventional sheet feeders take up a considerable amount of space and are quite expensive. A typical sheet feeder for document assembly will comprise a plurality of individual feed stations, positioned side-by-side and each containing a stack of sheets to be fed. Conventionally, the stacks are horizontally disposed (the sheets are stacked vertically on top of one another), and the sheets are fed onto a conveyor which has fingers to separate the individual assembled documents from one another. In the conventional system a large vacuum source is required, since all feed stations use vacuum to simultaneously extract a sheet from each stack and place it on the conveyor. This creates a considerable peak load on the vacuum system, requiring expensive components.
The present invention offers a considerable improvement over conventional feeders by arranging the stack of sheets on a steep incline, which take much of the weight of the stack off the sheet being fed and minimizes the tendency for two sheets to stick together. By arranging the sheets on an incline, the individual feed stations can be placed closer together so that the resulting feeding apparatus will have a smaller footprint. Also, by allowing the feed stations to be placed closer to one another, it is possible to employ an alternating feed sequence where even stations feed first, followed by odd stations. By sequencing the operation, peak loads on the vacuum system are greatly reduced, thereby reducing the cost of the system.
When two sheets do stick together, as can occur when two sheets are adhered to one another by static electricity, it is important to have a mechanism for sensing this, so that the feeding cycle can be terminated. The problem is one of discriminating between the thickness of a single sheet versus multiple sheets. As noted above, paper stock comes in a wide variety of different thicknesses and surface textures. Conventional sheet feeding equipment has had problems discriminating multiple sheets for misfeed detection purposes, due to the fact that sheet thickness can vary so widely.
The present invention addresses the misfeed problem by providing a highly accurate analog sensing system which employs a mechanically coupled occluding means in the path of an optical beam. The invention is capable of sensing paper thickness with a high degree of accuracy and is capable of being electronically programmed to discriminate a single sheet from a missing sheet or unwanted multiple sheets.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.